On September 1984 Apple released the follow-up to the Macintosh, addressing one of the major complaints of potential buyers.

Sold for USD 3,300 (or 3200, according to some sources), the Macintosh 512K was nicknamed “Fat Mac” for its increased (four-fold) RAM memory but was otherwise identical to the original Macintosh, as one can see from the dual-purpose motherboards.

In an 1984 interview in Byte with three of the original designers of the Macintosh, Jef Raskin actually revealed that the expansion was planned since the beginning and wasn’t an afterthought.

At the question

You started with 64K bytes and it was released with 128K bytes, and there is constant talk of a half-megabyte Mac. When did a half megabyte creep into the design philosophy?

Raskin answered:

Very early on Burrell [Smith, the motherboard designer, nda] pointed out that it’s very easy to make a design, once you had the 68000 in place, where you could just take out 64K-bit chips and put in 256K-bit chips. I’ve always believed that you just simply take the largest chip that is economically feasible to use in terms of the memory, and if they’re bit-wide chips and you use 8 or 16 of them, then that should be the size of your memory. […] Burrell loves designing for it, software part portion had no trouble handling that, and it was was very clean. When the 256K-bit chips come you just plug in all those and everything runs just about the same.

And things ran just about the same, but better: the 512k greatly improved application usage and even some operations helping avoid issues such as the “Disk Swapper’s Elbow”.

It was discontinued in April 1986, replaced by the 512Ke which had bigger ROMs (128K instead of 64) and used more capacious 800KB floppy disks.